Heat Recovery Method and Apparatus

ABSTRACT

A hot water heater or similar heating device includes equipment for pre-cooling hot flue gas while preheating water for the water heater. It further includes a heat and mass exchanger for transferring heat and water from the pre-cooled flue gas to combustion air for the hot water heater. The pre-cooler may comprise a separate device or may be incorporated as part of a condensing water heater. The heat and mass exchanger may use membranes having condensing sides and evaporating sides, which allow water to pass from the condensing sides to the evaporating sides. It may further comprise troughs for wetting the membranes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and apparatus for improving theefficiency of systems such as gas hot water heaters by a process ofhumid flue gas heat recovery.

2. Discussion of Related Art

Most industrial processes use large quantities of fuel and electricitythat ultimately produce heat, much of which is wasted either to theatmosphere or to water. A variety of methods and equipment have beendeveloped to reuse some of this waste heat. This may save up toapproximately 20 percent of a facility's annual fuel bill and, in someinstances, reduce pollution emissions and plant maintenance. However, inother applications it may increase pollutants (e.g., preheatingcombustion air increases combustion temperatures which can increase NOx)and maintenance.

Waste heat's usefulness is determined by its temperature; the higher thetemperature the higher the quality or value. Most waste-heat-recoverydevices transfer heat from a high-temperature effluent stream to alower-temperature input stream. This can either increase the temperatureof the input stream, or change the input stream from a liquid to avapor, as in a water heater or boiler. All these devices can be broadlycategorized as heat exchangers.

Heat recovery equipment must take into account temperature, pressureranges, corrosiveness of the effluent and input streams, presence ofmaterials that could foul the heat exchange surfaces, and thermalcycling. Extreme values of any of these may dictate the use of specialmaterials and design, resulting in high implementation costs. Inaddition, the waste-heat source and the site for use of the recoveredheat should be reasonably close.

In a process that requires heat as input, using waste heat can displacefuel or electricity that would otherwise have to be purchased. Ofcourse, the waste heat recovered has to account for enough fuel savingsto make up for capital and operational costs of the heat-recoveryequipment.

In actual industrial facilities several processes may exist that coulduse the waste heat. The higher the temperatures of the waste heat themore potential gain that would be available.

This list identifies process input and output characteristics that canhelp give a relative sense of possible energy savings from waste-heatrecovery.

-   -   The greater the temperature, flow rate and moisture content, the        greater the quantity of heat in the stream.    -   The proximity of waste-heat to possible processes that could        uses this heat influences total energy savings, due to heat loss        in fluid transit from the source, and the energy required to        move the fluid.    -   Latent heat from the condensation of moisture in exhaust gas can        be significant; however condensation is often undesirable due to        low condensing temperatures and the potential for corrosion        down-stream of the heat-recovery device.

Waste-heat recovery can save up to 20% of the energy costs in industrialfacilities, as noted earlier. Most of the energy savings will affectfuel consumption, however, electrical heating of ancillary equipment mayalso be affected.

Most gas or oil water heaters on the market today are simply a hot waterstorage tank, cold water in, hot water out, insulated tank, combustor, agas or oil regulator valve and a center flue gas to hot water heatexchanger tube using natural buoyancy of hot flue gas. See, for example,FIG. 3A (prior art). Because the exhaust gases can still be hot whenleaving the system there are known several patents that add ambient airto the flue gas to cool down the flue gas requiring a blower andadditional ducting: see U.S. Pat. Nos. 5,697,330 and 7,159,540.

To increase efficiency, a blower can also be added to overcome anypressure drop created by an internal baffle in the heat exchanger tube,see U.S. Pat. No. 7,032,543.

There is at least one known flue gas treatment that removes carbonmonoxide with the use of a catalyst converter, see U.S. Pat. No.7,055,465.

U.S. Pat. No. 4,175,518 describes a water heating system with apreheater which utilizes hot flue gases to preheat not only incomingcold water, but also for recirculating and preheating water from thestorage tank of the system. Preheaters for hot water heating systems arenot per se new. It has been suggested many times in the past that hotflue gases may be used in order to preheat incoming water for a hotwater tank.

Today a popular water heater on the market is the Condensing WaterHeater, (CWH). See, for example, FIG. 3B (prior art). The drawback tothe CWH is that often hot water use is sporadic so that small amounts ofcold water are brought in and the burners are on for a much longer timethan the water coming in. This means there is often little to noincoming water to preheat the hot exhaust gas and thus little efficiencygain. These condensing water heaters apparatus and methods are wellknown for adding additional heat exchange with cold water entering or inthe tank cooling the flue gases to below their dew point temperaturecausing water to condense out (see U.S. Pat. Nos. 4,541,410 and4,651,714). However, in such a water heater the hot exhaust gas iscooled with cold or cool water that is not always available and maydischarge hot flue gas from the heat exchanger with insufficient coolingfor condensing preventing high efficiency.

The water heaters of the conventional type described above often haveless than ideal levels of combustion efficiency and undesirably highlevels of emitted pollutants such as Nitrogen Oxide and Carbon Monoxide.

SUMMARY OF THE INVENTION

The present invention improves the thermal efficiency of a water heater(or similar heating equipment) whenever the equipment is in operation,and not just when cold water is available to cool flue gases. Providinghigh efficiency is accomplished by reducing the flue gas temperaturebelow its dew point temperature or lowering its enthalpy to near theoutdoor air temperature enthalpy whenever in operation. This isaccomplished by a second stage Heat and Mass Exchanger (HMX) that heatsand humidifies input combustion air while cooling and condensing fluegas. A first stage heat exchanger or condensing hot water heaterinitiates the flue gas cooling and/or condensing process. The condensingstarts at a much higher temperature than previous water heaters due tothe added humidity in the combustion air and thus a higher dew pointtemperature of the flue gas.

The present invention lowers the Nitrogen Oxide and Carbon Monoxide ofthe combustion process by increasing the absolute humidity of thecombustion air. Reducing Nitrogen Oxide and Carbon Monoxide becomes aside benefit of the high humidity created to lower flue gas enthalpy.

The present invention further transports water from the HMX condensingchannels through membranes to the evaporative channels directly. Thisprovides an efficient means to transfer heat and transfer water from thecondensing side to the evaporation side with little or no water makeupneeded.

The invention delivers the desired amount of water at any time throughthe unique design of a trough.

The present invention provides an improved heat recovery process andapparatus for transferring “waste” heat from the exhaust gases for awater heater, boiler, furnace, or other heating equipment, where thewaste heat is used to heat or preheat water, air or some other substancethat is needed (such as water in a water heater) and to heat andhumidify the air supplied to a combustion chamber, through the use offlue gas latent heat.

The heat recovery method of the invention makes use of pre-cooling and aheat and mass exchanger that create the following advantages:

-   -   1. High Efficiency all the time, rather than just when a large        amount of hot water is being used;    -   2. Raised condensing temperature, which results in higher        temperature condensing waste heat to heat with;    -   3. Less pollution due to moisture in combustion;    -   4. Higher efficiency heat and mass transfer in the HMX;    -   5. Trough reservoir that delivers correct amount of water all        the time;    -   6. Better burning;    -   7. Can be located outside water heater, which allows        retrofitting.

The present invention provides heat recovery apparatus comprising acounter flow heat and mass exchanger (HMX) for a water heater, boiler,furnace, and other heating equipment.

The HMX apparatus was specifically designed to be an efficient coolerfor the flue gas, and simultaneously to saturate the input combustionair before this air enters the combustion apparatus. In addition andbecause of the higher efficiency through the HMX, pollution isdramatically reduced due to the high levels of water vapor creating amore even burning process during combustion. Additional water will notbe needed, because the proposed heat recovery process and apparatusconstantly reclaims water condensed in the flue gas.

Drain water can come from two sources: water condensed from the flue gasin the water heater or heat exchanger by heating or preheating water;and water condensed from the flue gas in the HMX (minus what isevaporated by the combustion air in the heat and mass exchanger). Athigher outdoor air temperatures the HMX may produce more evaporationthan condensing, creating a need for water to be added.

The HMX has counter flowing evaporation and condensing channels onopposite sides of a heat exchange membrane, which:

1) allows heat transfer through the voids in the membrane filled withwater, due to the thin polymer wick construction (or other suitablematerials), but minimizes heat transfer laterally along the plate;2) allows mass (water) transfer from condensing channels to evaporativechannels through the membrane, due to the membranes structure or abilityto hold water by capillarity of membrane construction.

The membrane also has perforations between the condensing side of themembrane and the evaporative side or the membrane in defined areas,providing water flow from the condensing channels to the evaporativechannels in which indirect evaporative cooling takes place. This directtransfer of water from the condensation side to the evaporation sidereduces the heat and mass transfer resistance. Along the plate the hotflue gas temperature is transferring both sensible and latent heat andcondensing in direct contact with water evaporating and heatingcombustion air. This makes for very efficient heat and mass transfer ascondensing on one side of the membrane and evaporation directly on theopposite on the other side of the membrane results in more directtransfer of sensible and latent heat.

This system works by continuous cycling of water, by evaporating it intothe combustion air stream while condensing it from the flue gas. Thiscycling of water is kind of like a heat pipe that evaporates andcondenses a refrigerant. In both cases energy is transferred from onesource to another through evaporation and condensing.

The HMX provides an indirect evaporative cooler having efficient wickingaction via a trough, allowing easy wetting of the surface area of thewet channels without excess water (which would cool the water ratherthan the air).

There are a couple ways to increase the efficiency of a gas hot waterheater. High efficient hot water heaters on the market today gain theirefficiency by preheating the cold water going into the water heater withthe exhaust gas leaving the water heater. When exhaust or now flue gasis cooled to a low enough temperature water vapor from burning the fuel,(oxidizing or combining the H with O₂ to get H₂O), partially condensesadding a significant amount of heat to the cold water coming in.Therefore even the latent heat (condensing water vapor), is used to heatthe water. Condensing water vapor adds a significant amount of heat tothe system considering that it takes 1 Btu to cool one pound of water 1°F. and 1040 Btu to condense that same one pound of water vapor. Addingthe latent heat has a significant effect on the thermal efficiency. Thisis all built into the internal design of water heaters according to thepresent invention.

The present invention significantly raises the dew point temperature ofthe flue gas making it possible to at a minimum preheat the hot waterand in many cases heat the hot water to its high temperature. Theproposed high efficiency heat recovery method starts the moment the hotexhaust gases pass through the counter flow heat and mass exchanger(HMX). It is not dependent on cold water that may be short-lived or notpresent, such as when bringing the water heater up to temperaturewithout any cold water being added.

Within the HMX low temperature flue gas has its energy transferred tothe combustion air through temperature and humidity exchange. The fluegas temperature is cooled to below its dew point temperature or moreimportantly lowering its enthalpy to near the input combustion airtemperature enthalpy whenever it is in operation.

Raising the combustion air temperature requires less heating of the airfuel mixture, and is therefore more efficient. What is more surprisingis that adding humidity to the combustion air will also reduce the fuelneeded. The added humidity increases the mass flow of the inputcombustion air at a higher temperature requiring less fuel to heat thehot water.

Water vapor has other positive effects. For example, it comprisespolyatomic molecules (three atoms H₂O as opposed to two atoms like O₂ orN₂), that can radiate and be radiated to. This ability to radiatereduces hot spots in the burning process giving more complete burningwith about half the amount of NOx, an endothermic or energy drainingreaction. This is similar to but better than an automobile engine thatuses a small amount of Exhaust Gas Recirculation or CO₂ plus H₂Orecirculation to lower its NOx. The higher efficient burning at lowertemperatures also decreases the carbon monoxide in the same way asreducing NOx.

The present heat recovery method and apparatus can also includeimplementing the Maisotsenko Cycle or M-Cycle (see paper: L. Gillan,“Maisotsenko cycle for cooling process”, Clean Air, 9 (2008) 1-18),which is also suited for the HMX, especially in the winter time, whenthe dew point temperature is low. This more thermally efficient processcools the input combustion air to near its dew point in the working airdry channels, and humidifies working air in its wet channels. This is inpreparation for cooling and condensing water from the combustion exhaustgas and further heating and humidifying the input air that is nowcombustion air.

The air psychrometric saturation line slopes such that cool air has agreater change in energy for a given humidity ratio change than athigher temperatures. This means that in a Humid Air Recovery (HAR) HMXthere will be more condensation than evaporation, producing distilledwater. The design of this HAR HMX takes into account the higher rate ofcondensation from the combustion exhaust gas than the water evaporatedby allowing condensate to pass directly through the plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a preferred embodiment of an HMXHumid Air Recovery (HAR) system for a gas hot water heater according tothe present invention.

FIG. 2A is a schematic diagram of a preferred embodiment of a HARsystem, using a membrane.

FIG. 2B is a schematic diagram of the embodiment of FIG. 2A, showingwater held by the membrane.

FIG. 3A (prior art) is a schematic diagram showing a conventionalnon-condensing water heater

FIG. 3B (prior art) is a schematic diagram showing a conventionalCondensing Water Heater (CWH)

FIG. 3C is a schematic diagram showing a hot water heater similar tothat of FIG. 3B, but with an HMX HAR system according to the presentinvention.

FIG. 4 is an isometric diagram showing a preferred embodiment of an HARsystem utilizing a counterflow HMX and troughs to provide water.

FIG. 5 is an isometric drawing showing the bottom of a trough of FIG. 4coated with an impervious coating.

FIG. 6 is an isometric drawing showing a stack of the troughs of FIGS. 4and 5 with overflow perforations

FIG. 7 is an isometric diagram of an HAR system utilizing a counterflowHMX and troughs, utilizing an end cap for the trough stack.

FIG. 8 is a schematic diagram showing a counterflow HMX according to thepresent invention.

FIG. 9 is a schematic diagram showing an HMX according to the presentinvention, wherein input combustion air first travels along dry sides ofmembranes.

FIG. 10 is a schematic diagram of an HMX similar to FIG. 9, with theinput combustion air eventually split into two streams and turned totravel counterflow.

FIG. 11 is a schematic block diagram of a system for providing acombustor with saturated combustion air according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION Reference Number List

-   1, 301 Input Combustion Air-   2, 302 Saturated Combustion Air-   3, 303 Combustor-   4, 304 Hot Flue Gas-   5, 305 Precooled (Warm) Flue Gas-   6, 306 Flue Gas Out (Cool)-   7, 307 Heat and Mass Exchanger HMX-   8, 308 Pre-cooler-   9 Water Heater-   10 Pump-   11 Cold Water In-   12 Cooling Water-   13 Heated Water-   14 Hot Water Out-   15 Cool Water to Tank-   16 Liquid (Condensate)-   17 Drain Water-   18 Trough-   19 Membrane,s (Plates)-   20 Fan-   22 Heat-   23 Evaporated Water-   24 Membrane Water-   25 Trough End Cap-   26 Dry (condensing) Side of Membrane-   27 Trough Overflow Perforations-   28 Channel Guide-   29 Impervious Coating-   30 Wet (evaporating) Side of Membrane-   32 Condensing Water Heater-   34 HMX Humid Air Recovery System-   35 Flue Gas Coil-   36 Tank Water-   37 Flue Gas Channels-   38 Combustion Air Channels-   39 Tank of Hot Water Heater 9-   40 Non-condensing Water Heater-   41 End Element-   42 Fuel-   300 Combustor System

FIG. 1 is a block diagram illustrating a preferred embodiment 34A of aHumid Air Recovery (HAR) water heater system, including a Heat and MassExchanger (HMX) 7 and a Pre-cooler 8 (in this case a pre-condenser) fora gas Hot Water Heater 9 according to the present invention. FIGS. 2Aand 2B illustrate the process.

In a preferred embodiment of HAR HMX system 34, Input Combustion Air 1is forced into Heat and Mass Exchanger (HMX) 7 by Fan 20, where it isheated and saturated with Water 24 becoming Saturated Combustion Air 2.Water 24 is heated by Precooled Flue Gas 5. The water may compriseCondensate 16 and/or an independent source of water such as shown inFIGS. 4-7.

Saturated Combustion Air 2 then enters Combustor 3 where it is combustedwith Fuel 42 and used to heat Cool Water 15 within Tank 39 of WaterHeater 9. The somewhat cooler Hot Flue Gas 4 then enters Pre-cooler 8where it is cooled to below its condensing temperature by Cooling Water12 (generally comprising Cold Water 11 and/or Cool Water 15), andbecomes Precooled Flue Gas 5 (aka warm flue gas). Thus, Hot Flue Gas 4preheats Cold Water 11 or heats Cool Water 15, and is itself cooled tobecome Precooled Flue Gas 5. Water condenses from Hot Flue Gas 4 andbecomes Condensate 16. Precooled Flue Gas 5 and Condensate 16, (if theflue gas is cooled below its dew point temperature), are provided to HMX7. Because Cold Water 11 may not be flowing at all times when WaterHeater 9 is calling for heat, Pump 10 may be provided to pull Cold Water11 or Cool Water 15 into Pre-cooler 8 (as Cooling Water 12). CoolingWater 12 is warmed within Pre-cooler 8 and returns to Tank 39 as HeatedWater 13.

Precooled Flue Gas 5 is further cooled in HMX 7 causing additionalcondensation (not shown). Condensate 16 may drain to Trough 18 (SeeFIGS. 4-7) in HMX 7 ensuring an adequate water supply when outdoorweather conditions are hot and dry. Excess water from Trough 18 becomesDrain Water 17 that is drained from the system.

In a typical non-condensing water heater, FIG. 3A (prior art), ColdWater 11 into Water Heater 9 is delivered through a Tube 31 where itstarts to mix in Water Heater 9 Tank 39 becoming Cool Water 15. InputCombustion Air 1 enters the bottom of the Water Heater 9 where it entersCombustor 3. The exhaust Flue Gas 4 passes through Water Heater 9,heating the water and leaving. Hot Water 14 leaves the tank for use.

In a typical Condensing Water Heater 32, FIG. 3B, (there are manyvarieties), Input Combustion Air 1 is directed into Combustor 3 throughan induced draft Fan 20 that pulls Hot Flue Gas 4 to the top of Flue GasCoils 35. Water 36 is heated, cooling the flue gas below its dew pointtemperature creating Condensate 17 that is collected and drained fromthe system.

A Condensing Water Heater 32 can replace Pre-cooler 8 in FIG. 2, suchthat the systems needs only HMX 7 as shown in HMX Humid Air Recovery 34Bin FIG. 3C. This eliminates the need for Pump 10.

FIG. 2A is a schematic diagram of a preferred embodiment of Humid AirRecovery, using a Membrane 19. Hot Flue Gas 5 provides Heat 22 in theform of higher temperature and condensate to Membrane 19 becoming FlueGas Out 6. Evaporated Water 23 is taken up by Input Combustion Air 1becoming Saturated Combustion Air 2. FIG. 2B is a schematic diagram ofthe embodiment of FIG. 2A, showing Water 24 held by Membrane 19 which isevaporated by Heat 22 to become Evaporated Water 23 into InputCombustion Air 1 becoming Saturated Combustion Air 2. Water 24 may alsobe wicked onto the surface of Membrane 19 facing Input Combustion Air 1(the wet side of membrane 19).

FIG. 3A (prior art) is a schematic diagram showing a conventionalNon-condensing Water Heater 40. FIG. 3B (prior art) is a schematicdiagram showing a conventional Condensing Water Heater (CWH) 32. Fuel in42 is not shown.

In a typical Non-condensing Water Heater 40 or CWH 32, the condensingtemperature of Hot Flue Gas 4 would be about 131° F. This is based onthe amount of water created by the oxidation of the hydrogen. Thistemperature is so low that it can only be used to somewhat preheat ColdWater 11 entering Water Heater 9 but typically not sufficiently heatCool Water 15 within Tank 39. Non-condensing Water Heater 40 uses about70% of the heat from Hot Flue Gas 4, as it passes straight up the centerof Tank 39.

CWH 32 does well when Cold Water 11 is entering Tank 39, as Hot Flue Gas4 takes a circuitous path 35 within Tank 39. The efficiency of CWH 32quickly drops off to about 80% efficiency or less when Cold Water 11 isnot entering Tank 39.

FIG. 3C is a schematic diagram showing a Hot Water Heater 9 similar tothat of FIG. 3B, but with an HMX HAR system according to the presentinvention. FIG. 3C shows the advantages of the present invention. Whenhumidity is added to Input Combustion Air 1 in HMX 7, SaturatedCombustion Air 2 is formed. The Hot Flue Gas 4 dew point temperaturerises to about 160° F. This temperature is typically hot enough toeither heat or preheat Cool Water 15. Hot Water Out 14 of Water Heater 9is often only set at about 130° F. so that it will not be too hot andcause burns at the water faucets. This allows the 160° F. condensingflue gas to sufficiently heat the water. In addition, because the wateris causing Hot Flue Gas 4 to condense, the latent heat of vaporizationcan heat a significant amount of water to a much higher temperature.

In order to better understand this Humid Air Recovery method, Table 1has four mathematical simulations: run1 a typical Water Heater 9; run2using only the HMX 7; run3 using a Pre-Cooler, (no condensing) with anHMX 7; and run4 using a Pre-cooler 8 with an HMX 7.

TABLE 1 Part 1 Combustion Air Outdoor Air In Dry Gas Burned Air Mass 1,Mass Out of HMX Flow Firing Temp Flow, 2, Humidity Rate, Rate, DryHumidity Enthalpy, lb/ Dry Ratio, Enthalpy, Description Run lb/minbtu/hr Bulb, F. Ratio btu/lb min Bulb, F. lb/lb btu/lb Water Heater 10.0869 110, 523 68 0.0080 25.05 1.63 68 0.0080 25.05 Only HMX Only 20.0869 110, 523 68 0.0080 25.05 1.63 150 0.2110 273.92 PreCool/HMX 30.0869 110, 523 68 0.0080 25.05 1.63 150 0.2110 273.92 PreCond/HMX 40.0869 110, 523 68 0.0080 25.05 1.63 150 0.2110 273.92 Part 2 Flue GasPre-Condenser Flue Gas Into Pre-condenser Water Flue Gas Out of WaterPreheated In to Dry Pre-condenser Pre-condenser Flue and Into HMX 12,13, Water Gas 5, Temp Temp Flow Cond 4, Ratio, Enthalpy, Temp Enthalpy,Water Water Rate, Run Temp, F. Temp, F. lb/lb: btu/lb Out, F. btu/lb In,F. Out, F. lb/min 1 131.7 400 0.1170 240.91 400 240.91 135 135 0.0 2163.2 400 0.3315 506.59 400 506.59 135 135 0.0 3 163.2 400 0.3315 506.59164 415.21 135 145 14.1 4 163.2 400 0.3315 506.59 158 350.89 135 14524.079 Part 3 Flue Gas HMX Water Drained Flue Gas Out of HMX Water MassVapor Flow above of Dry Oxidized Total Outdoor Humidity Flue Comb WaterAir 17, 6, Ratio, Enthalpy, Gas, H2O, Cond, Exhaust, Drained, Run Temp,F. lb/lb btu/lb lb/min lbw/min lbw/min lbw/min lbw/min Efficiency 1400.0 0.1170 240.91 1.55 0.168 0.00 0.17 0.000 82.0% 2 146.1 0.1856244.03 1.55 0.168 0.23 0.27 −0.106 81.7% 3 129.2 0.1082 152.06 1.550.1679 0.35 0.16 0.014 89.5% 4 107.4 0.0545 86.17 1.55 0.168 0.43 0.070.097 95.0%

In Water Heater only, run1, the flue gas is cooled only in Water Heater9 and exits the system at about 400° F. See FIG. 3A for a typicalconfiguration. The condensing temperature of the flue gas is about 131°F. This results in an efficiency of about 82%, high for a Non-condensingWater Heater 40, and typically would require Cold Water 11 entering theTank 39.

In run2 with the HMX only, (not shown but for the case where an HMX isconnected to a Non-condensing Water Heater 40) more water is evaporatedthan condensed because of the cooling of the Hot Flue Gas 4 down from400° F. and condensing starts at about 160° F. As with all heatexchangers the energy removed on one side must equal the energy gain onthe other. On the cooling side the Input Combustion air 1 will be heatedbut most of the heat gain will be through evaporation of water whilebecoming Saturated Combustion Air 2. In this run more water isevaporated into the combustion air stream than can be condensed from theflue gas due to desuperheat of the Flue Gas 5. Nothing is gained as faras efficiency is concerned as the flue gas enthalpy leaving the systemis about the same as run1, lower temperature but saturated. Over all theefficiency is about the same as run1 or maybe a little less, 82%. Whatis needed in this embodiment is cooling of Hot Flue Gas 4 before the HMX7, or putting flue gas heat to use such as for heating the water in theWater Heater 9 or Cold Water In 11.

Run 3 demonstrates that when Pre-cooler 8 cools Flue Gas 5 to just aboveits condensing temperature the maximum efficiency is limited to about90%.

In run 4 the Hot Flue Gas 4 is pre-condensed in Pre-cooler 8 to about158° F. This results in an efficiency of about 95%. As mentioned earlierthe water can now be heated at much higher temperature with the flue gasat 158° F. rather than 131° F. This is the configuration of FIG. 1 orFIG. 3C.

As can be imagined, this heat recovery method can be used on otherdevices such as furnaces, boilers, and other applications that haveeither an internal need for a heat above 140° F., to heat water in thehot water heater case shown, or to heat another fluid. FIG. 11illustrates a more general system.

HMX 7 can be as simple as a heat and mass exchanger that is able to havecondensing on one side of the plate and evaporation on the other. Ofcourse on the evaporation side there must be a means to distribute waterfrom the condensing side or from another source across the plate (e.g.,wicking, spraying, gravity delivery, etc.) On the condensing side theremust be a means to collect the water and either deliver it to theevaporation side or drain it away.

One preferred HMX method was shown in FIG. 2A where Membrane 19separates Pre-cooler Flue Gas 5 from Combustion Air 1. Water, e.g.Condensate 16, passes through Membrane 19 to the Combustion Air side 30and Evaporated Water 23 is added to Combustion Air 1. Membrane 19 ismade of a wick material that will hold Water 24, see FIG. 2B, withcapillary action (like water absorbed in a towel). The water in the wickmembrane separates the flue gas from the combustion air. In a preferredembodiment a stiff wicking material is used such as a polyester spunbondmaterial that is flat bonded. This has several advantages, e.g. there islittle heat resistance through Membrane 19, no pump is needed to movewater from one side to the other, and the water condensing on the fluegas side becomes the water on the evaporating side. In this way no heator mass transfer is lost in gathering water, pumping water to the otherside and evaporating it.

FIG. 4 is an isometric diagram showing a preferred embodiment of a HARsystem utilizing a counterflow HMX 7 with Troughs 18 to provideadditional water as needed. In this embodiment, Membranes 19 areseparated and supported by Channel Guides 28, which form channels 37,38. Flue Gas Channels 37 direct Pre-condensed Flue Gas 5 on Condensingside 26. Combustion Air Channels 38 direct Input Combustion Air 1 onEvaporation Side 30 of each Membrane 19 in counterflow. WarmPre-condensed Flue Gas 5 becomes cool Flue Gas Out 6. Input combustionAir 1 becomes Saturated Combustion Air 2.

Membranes 19 are attached to Troughs 18 having Trough OverflowPerforations 27 to drain excess water from one Membrane to the nextMembrane down. Troughs 18 insure that Membranes 19 are always wettedduring startup and when it is hot and dry out, for example with addedCondensate 16 from Pre-cooler 8. Trough 18 also can collect excesscondensing Water 24 from Flue Gas 5 that is not evaporated into InputCombustion Air 1 as it is condensed on Membrane 19.

FIG. 5 is an isometric drawing showing the bottom of Trough 18 coatedwith an Impervious Coating 29 to prevent water from dripping throughexcept via Trough Overflow Perforations 27.

FIG. 6 is an isometric drawing showing Trough Over Flow Perforations 27in a stack of Troughs 18. As each Trough 18 fills with water, the TroughOver Flow Perforations 27 allow water to drain to the next Trough below.The bottom Trough will allow the water to drain from the system as DrainWater 17.

FIG. 7 is an isometric drawing showing an HMX system 7 configured withTrough End Caps 25 that prevent the water in Troughs 18 from flowing outthe ends.

There are many heat and mass exchanger configurations that can be used.FIG. 8 is a schematic of a counter flow HMX. Input Combustion Air 1passes along saturated Membranes 19 in Combustion Air Channels 38becoming Saturated Combustion Air 2. It is heated by Pre-condensed FlueGas 5 in Flue Gas Channels 37, and causing water to condense out as itpasses along the opposite side of Membrane 19. Flue Gas 5 becomes FlueGas out 6.

Membrane 19 could be an impervious plate with other means to distributethe condensate water from one side of the plate to the other forevaporation.

To create lower temperatures it maybe desirable to use the M-Cyclewherein Input Combustion Air 1 first travels along Dry Sides 26 ofMembranes 19 where it is cooled towards its dew point temperature, asshown in FIG. 9. It is then turned to travel counterflow across the WetSides 30 of Membranes 19 and becomes Saturated Combustion Air 2 as itpicks up heat from both the Input Combustion Air 1 and the Pre-condensedFlue Gas 5 through Membrane 19.

Another schematic of an M-Cycle type of HMX which is useful inembodiments of the present invention is shown in FIG. 10. InputCombustion Air 1 first travels along Dry Sides 26 of Membranes 19 whereit is cooled towards its dew point temperature as in the embodiment ofFIG. 9. It is then split into 2 streams and turned to travel counterflowacross the Wet Sides of Membranes 30 and becomes Saturated CombustionAir 2 as it picks up heat from both the Input Combustion Air 1 and thePre-condensed Flue Gas 5.

FIG. 11 is a schematic block diagram of a system 300 for providing aCombustor 303 with Saturated Combustion air 2 according to the presentinvention. Combustor 303 generates Hot Flue Gas 4, which is cooled byPre-cooler 8, generating Warm Flue Gas 5. Pre-cooler 8 utilizes a fluid312 to accomplishes precooling.

HMX 307 warms and humidifies atmospheric air 1 (with Warm Flue Gas 5 andLiquid 24) to produce saturated combustion air 2 for combustor 303. CoolFlue Gas 6 is generally vented into the atmosphere.

What is claimed is:
 1. A heat recovery method comprising the steps of:a) providing a first heat and mass transfer membrane having a condensingside and an evaporating side; b) wetting the evaporating side of theheat transfer membrane with an evaporative liquid; c) passing hot humidgas along the condensing side of the heat transfer membrane; d) passingcool dry air along the evaporation side of the heat transfer membrane;e) cooling the gas along the condensing side of the membrane byevaporating the evaporative liquid into the gas passing along theevaporation side of the membrane and saturating the gas passing alongthe evaporation side of the heat transfer membrane; and f) passingcondensed evaporative fluid through the membrane from the condensingside to the evaporating side.
 2. The method of claim 1 wherein theevaporative fluid is water.
 3. The method of claim 1, further includingthe steps of: providing a plurality of heat and mass transfer membraneshaving condensing sides and evaporating sides, and placing the membranesadjacent to each other such that evaporating sides of membranes faceeach other and condensing sides of membranes face each other; applyingsteps (b)-(f) to the membranes; providing channels between the membranesto guide air flow.
 4. The method of claim 3 wherein the evaporativefluid is water.
 5. The method of claim 3, further including the steps ofproviding troughs for liquid at the edges of membranes and wickingliquid from the troughs to the evaporating side of the membranes.
 6. Themethod of claim 5, further including the steps of providing overflowperforations through troughs and draining liquid through theperforations.
 7. The method of claim 5, further including the step ofcollecting excess condensate water.
 8. The method of claim 4 furtherincluding the steps of: providing a pre-cooler; passing hot flue gasfrom a hot water heater through the pre-cooler to provide hot humid gasfor step (c); preheating water with the pre-cooler; providing thepreheated water to the water heater; providing the saturated gas fromstep (e) as combustion air for the water heater.
 9. The method of claim8 wherein the pre-cooler is a pre-condenser.
 10. Apparatus for heatrecovery comprising: a heat and mass exchanger including— a plurality ofmembranes having condensing sides and evaporating sides and placed in astack with condensing sides facing condensing sides and evaporatingsides facing evaporating sides, wherein the membranes allow evaporativeliquid to pass through the membranes from condensing sides toevaporating sides; wetting apparatus for wetting evaporating sides ofmembranes with an evaporative liquid; condensing-side channels formedbetween adjacent condensing sides of membranes; evaporating-sidechannels formed between adjacent evaporating sides of membranes; a fluegas providing element for flowing hot humid gas through condensing-sidechannels and transferring heat to the membranes; a combustion airproviding element for flowing cool dry gas through evaporating-sidechannels and cooling membranes, cooling flue gas, and saturatingcombustion gas.
 11. The apparatus of claim 10 wherein the membranesinclude a wicking structure for spreading evaporation fluid overevaporating sides and a capillary transfer structure for passingevaporative fluid through the membranes.
 12. The apparatus of claim 11,further comprising troughs at the edges of membranes for providingevaporative fluid to the wicking structures.
 13. The apparatus of claim12 wherein the troughs further include overflow perforations forallowing evaporative liquid to drain.
 14. The apparatus of claim 11,further comprising: a pre-cooler having a exhaust-gas channel forcooling flue gas from a hot water heater and providing it to the fluegas providing element; and a water channel for preheating water andproviding it to the hot water heater, and wherein combustion air fromthe HMX comprises combustion air for the hot water heater.
 15. Theapparatus of claim 14 wherein the pre-cooler is a pre-condenser.
 16. Ahot water heater comprising: a tank for water; a source of cold waterfor providing water to the tank; a combustor for heating the water inthe tank, the combustor configured to intake combustion air and fuel andoutput hot flue gas; a pre-cooling mechanism for heating water andcooling hot flue gas to output pre-cooled flue gas; and a heat and massexchanger including— a plurality of plates having condensing sides andevaporating sides and placed in a stack with condensing sides facingcondensing sides and evaporating sides facing evaporating sides; wettingapparatus for wetting evaporating sides of plates with an evaporativeliquid; condensing-side channels formed between adjacent condensingsides of plates; evaporating-side channels formed between adjacentevaporating sides of plates; an element for flowing pre-cooled flue gasfrom the pre-cooling mechanism through condensing-side channels andtransferring heat through the plates to the evaporation side; an elementfor flowing air through evaporating-side channels, saturating the airflowing through evaporating-side channels and providing it as humidcombustion air for the combustor.
 17. The hot water heater of claim 16wherein the pre-cooler is a pre-condenser.
 18. A device for providingwarm humid combustion air to a combustor comprising: a pre-cooler; aconduit for providing hot flue gas from the combustor to the pre-cooler;a conduit for providing a fluid to the pre-cooler; wherein thepre-cooler includes apparatus for cooling the hot flue gas with thefluid while warming the fluid and providing warm flue gas; a heat andmass exchanger (HMX); a conduit for providing input combustion air tothe HMX; a conduit for providing the warm flue gas from the pre-coolerto the HMX; a conduit for providing a liquid to the HMX; wherein the HMXincludes apparatus for warming the input combustion air with the warmflue gas and humidifying the atmospheric air with the liquid to producewarm, humid combustion air; and a conduit for providing the warm, humidcombustion air to the combustor.
 19. The device of claim 18 wherein thepre-cooler is a pre-condenser.
 20. The device of claim 19 wherein thefluid is water and further comprising a hot water heater having a tankconfigured for heating water in the tank with the combustor and furtherincluding a conduit for providing the warmed water to the hot waterheater tank.